In order to achieve warm white and neutral white luminous colors, metal halide discharge lamps generally contain sodium. For example, U.S. Pat. No. 3,575,630 describes a lamp which contains a metal halide fill which includes the elements Na, Tl and Zr, and which has a warm white luminous color. A further example is the lamp described in EP-A 883 160. This lamp has a metal halide fill which includes the elements Na, Sc, and other constituents, such as Mn. This lamp is dimmable.
Metal halogen discharge lamps having a discharge vessel made from glass and a sodium-containing fill are known to have the drawback of sodium diffusion through the discharge vessel, which reduces the service life of the lamps. The sodium diffusion has to be reduced by means of additional measures, for example shielding of the supply conductor in the vicinity of the discharge vessel, which increases the production costs of the lamp. A further drawback of sodium-containing metal halide discharge lamps is their relatively low color rendering. An Na—Sc-containing metal halide discharge lamp with a neutral white luminous color has, for example, typical values for the general color rendering index Ra=70 and special color rendering index R9=0.
U.S. Ser. No. 09/499,099, which corresponds to DE-A 199 07 301 describes a metal halide fill which includes Mn but does not include Na for metal halide discharge lamps in order to obtain warm white and neutral white luminous colors. Substitution of sodium eliminates the additional measures for reduction of sodium diffusion in the lamps which are filled with this fill. Furthermore, the lamps with the Mn-containing fill achieve high values for the color rendering, with Ra>95. However, the light yield and lamp output are relatively low. For example, at a 250 W inductor, which is also used for Na high-pressure vapor lamps, the lamp output is typically 240 W.
Finally, DE-A 35 12 757 has disclosed a fill for metal halide lamps which contains a metal silicide, such as V5Si3. In addition, the fill contains rare earth halide or Sc halide and the corresponding rare earth oxyhalide and/or Sc oxide. The silicide in this case acts as a halogen getter.
It is known that incandescent lamps and halogen incandescent lamps, which are Planckian radiators, can be dimmed without problems. However, if metal halide discharge lamps with a reduced lamp output are operated (cf. the abovementioned EP 883 160), their color locus moves away from the Planckian locus. The lamps lose their white luminous color and the color rendering deteriorates.
FIG. 1 (prior art) shows a color locus diagram for a metal halide lamp which is capped on two sides and has an output of 250 W and a neutral white fill (HQI-TS 250W/NDL produced by OSRAM) as an example, which has an Na-containing metal halide fill. The output of the lamp was reduced to half its light flux in stages approximately from 250 W to 160 W at an electronic ballast. As the output decreases, the color locus of the lamp migrates out of the region of the isotemperature lines. As a result, the lamp becomes increasingly greenish.